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RE: Horner's Pachy Lumpin' - Your Thoughts?



Zach Armstrong wrote:

> Tracy Ford, in Prehistoric Times issue #73 "How to Draw Dinosaurs-Spiky 
>Pachycephalosaurs?" commented that there are known Pachycephalosaurus>skulls 
>with spikes, not nobs, which means no loss of horns nor "absorbtion" 
>of horns is needed. One of the skulls was reported to have been found by 
>Mike Triebold. Ford reports that the Black Hills Institute has a spiked 
>Pachycephalosaurus skull. I am not sure if this is the same as Ford's 
>drawing of the "Sandy Site" Pachycephalosaurus of that same issue, but a 
>photo of the Sandy Site skull in all its spiky glory can be found here: 
>http://www.wmnh.com/wmtr0000.htm
>
> I should also point out that in PT #75, there is a 
>Pachycephalosaurus>skeletal done by Greg Paul that has long spikes, too. I 
>am not sure of his>reasoning on this, but I'm sure he has a good one :)
>
> If these skulls indeed are referable to Pachycephalosaurus, that means the 
>"Dracorex" juvenile would not have needed to lose any horns or re-absorb 
>them at all. Also, as far as I know, Stygimoloch is only known from 
>fragmentary skull which could explain the differing spike counts.

This is not true. For one thing, the squamosal horns of the Sandy Site 
_Pachycephalosaurus_ are clearly much shorter than those of _Stygimoloch_, 
whose squamosal horns are about a third again as long. So there would *still* 
be horn-absorption to a degree not seen in any extant terrestrial tetrapod. 
Second, contrary to what Zach says, a whole entire horn would also *still* have 
to be lost, as _Dracorex_ possesses four, _Stygimoloch_ three. From the 
_Dracorex_ description_:

"We have been able to determine, based on comparison with other documented 
specimens of _Stygimoloch spinifer_ (MPM 7111 and MPM 8111), and two 
undescribed specimens in private collections, that _S. spinifer_ consistently 
has these enlarged spikes coupled with an incipient, laterally compressed dome, 
made up of only the frontals and parietal."

_Stygimoloch_ is *not* known only from the fragmentary type specimen. Other 
specimens confirm that this taxon invariably possesses three horns. In Horner's 
senerio, the loss of a whole horn during ontogeny is inevitable.

Tim Williams wrote:

> Firstly, Bakker &c in the _Dracorex_ description are more guarded than
>you are on this matter of cervical fusion indicating a "young adult"
>individual. To quote directly from the _Dracorex_ description:
>
> "Although the specimen is considered to represent a young adult,
> we believe, based on the beginning of coosification [sic] of
> mid-cervical arch with centrum, that the animal was probably near
> maturity."
>
> So their exact words are: "beginning of coossification" (which is not the
>same as "strongly fused"), and "probably near maturity" (which is not
>as assertive as "a young adult at the very least".)

Poor choice of words on my part. I still, however, feel that the cervical 
fusion may be important somehow; Horner still should have at touched upon it 
and attempted to provide some sort of alternative explanation, in my opinion.

This disregard of potentially (or, in some cases, quite obviously) important 
data has been a consistency of Horner's for a long time. Rather than pinpoint 
conflicting data and explain it, debunk it, or find an alternative explanation, 
Horner simply pretends it isn't there, which tends to get annoying.

Just a general observation.

>> There is no known extant terrestrial tetrapod that grows in
>> the manner that Horner postulates for North American
>> pachycephalosaurids. The growth of the squamosal horns from
>> small, to large, to disappearing altogether (and losing a
>> whole frikkin' horn along the way), is unheard of in extant
>> mammals, birds, and 'reptiles'.
>
>
> This may or may not be true for extant tetrapods; I don't know. But the
>skull of _Triceratops_ apparently underwent this kind of transformation
>(including erosion), as Horner &c explicitly point out:
>
> "The squamosal horns and nodes of _Pachycephalosaurus_ underwent
> a similar transformation to the epiparietal and episquamosal
> elements of _Triceratops_ [16:figure 2] that (1) grew from
> diminutive to large triangular-shaped ornaments; (2) eroded as
> they reduced dorsoventrally and (3) flattened and lengthened as
> they merged onto the edge of the parietal-squamosal frill."

I'm aware of this, and if you read the rest of my post, you'll see where I 
discussed it. I wrote, and I quote in full:

"...The attempted use of a cassowary, which does not undergo ontogenetic 
reversals and reabsorbtions of cranial features during growth, to justify such 
in pachycephalosaurids is preposterous, counter-intuitive, and nonsensical. 
Exactly the same can be said for his further use of _Triceratops_ ontogeny as 
evidence in support of his 'pachy proposals', this time in the actual 
peer-reviewed paper (Horner and Goodwin 2009, of course). He cites the 
reabsorbtion of the epoccipitals in this taxon as an already documented example 
of the reabsorbtion and reversal in growth of cranial features in dinosaurs, 
lending credibility (in his mind) to his hypotheses on pachycephalosaurid 
cranial ontogeny. The problem is that the epoccipital bones are not horns. 
There is no reason to believe that the ontogenetic changes in these bones would 
likewise go for horns as well. The structures (pachycephalosaur horns and 
ceratopsian epoccipitals) are not even homologous. Rather, we have numerous 
examples, from both extant and extinct taxa, that demonstrate that horn 
reversal during ontogeny is not at all normal or natural for any terrestrial 
tetrapod. We can even use the _Triceratops_ analogy as evidence *against* 
Horner's proposals, for although the brow and nasal horns of _Triceratops_ 
change shape significantly during ontogeny, there are no reversals or 
reabsorbtions. In terms of size, the horns just keep getting bigger and bigger 
until they are at their longest and largest in fully grown adults."

...just the opposite of what Horner claims for pachycephalosaurids.

I will add to my original post that the restructuring and/or loss of the 
supratemperal fenestrate during ontogeny is *completely unknown* in *any* 
diapsid.

As I stated in my original post, I feel it is most parsimonious to conclude 
that pachycephalosaurids did *not* grow in this manner, as there is no extant 
analogue whatsoever, and even if one rejects *all* of the other 'con' 
arguments, the EBP still clearly argues against Horner's proposals, as no bird, 
crocodilian, or lepidosaur shows anything even remotely similar. Falsification 
trumps parsimony (and the EBP), but therein lies my point: I do not believe 
that Horner has falsified anything, and consider his case to be very weak and 
hole-filled. The view that these specimens represent three different taxa 
better explains the data we currently have than does the view that these 
represent merely three different ontogenetic stages of one taxon, as many 
aspects of the latter outright preposterous at worst.

Horner would, perhaps, do well to fallow the advice of a certain bearded 
individual whom shall remain nameless:

"Only champion heresies if they fit the facts better than the orthodoxy".

Hopefully, however, those who are reading this will realize that *I remain 
open-minded on this issue*: I am merely saying that Horner's going to have to 
do a *LOT LOT LOT* better than what he has done thus far to convince me that 
his interpretation is correct, and I have provided some counter-arguments for 
his claims.

I will add that I find it very distressing indeed that radical, controversial 
claims such as Horner's, that were just published a few months ago and *have 
not even been verified* (extra emphasis there), are being swallowed up by other 
authors as the current status quo and are even being applied to other taxa! 
(Talking about, of course, of course, the Longrich et al. _Texacephale_ paper.)

~ Michael

----------------------------------------
> Date: Thu, 7 Jan 2010 18:31:38 -0800
> From: tijawi@yahoo.com
> Subject: RE: Horner's Pachy Lumpin' - Your Thoughts?
> To: dinosaur@usc.edu
> CC: tehdinomahn@live.com; tijawi@yahoo.com
>
> Michael Erickson wrote:
>
>
>> I certainly don't buy it.
>>
>> For starters, the cervical vertebrae of the type specimen
>> of _Dracorex_ show haemal arches that are strongly fused
>> onto their respective centra. This is at least indicative
>> that the _Dracorex_ holotype was near maturity, a young
>> adult at the very least. Horner's case would have been much
>> more convincing had he had mentioned this; he instead
>> chooses to blatantly ignore it, not even *attempting* an
>> explanation - which puts a severe damper on his
>> credibility.
>
>
> I don't want to dwell too much on this. I'm keeping an open mind on this 
> particular issue - although at the moment I strongly lean toward Horner &c's 
> interpretation. However, a few issues are worth airing.
>
>
> Firstly, Bakker &c in the _Dracorex_ description are more guarded than you 
> are on this matter of cervical fusion indicating a "young adult" individual. 
> To quote directly from the _Dracorex_ description:
>
> "Although the specimen is considered to represent a young adult,
> we believe, based on the beginning of coosification [sic] of
> mid-cervical arch with centrum, that the animal was probably near
> maturity."
>
> So their exact words are: "beginning of coossification" (which is not the 
> same as "strongly fused"), and "probably near maturity" (which is not as 
> assertive as "a young adult at the very least".)
>
>
> Secondly, the morphology of the cervicals, especially the degree of 
> coossification, may be influenced by the need to support a very heavy skull - 
> even in juveniles and subadults. I suspect we need more data of the 
> ontogenetic development of the cervical vertebrae in pachycephalosaurs to 
> assess at what stage in ontogeny the cervicals underwent fusion.
>
>
>> There is no known extant terrestrial tetrapod that grows in
>> the manner that Horner postulates for North American
>> pachycephalosaurids. The growth of the squamosal horns from
>> small, to large, to disappearing altogether (and losing a
>> whole frikkin' horn along the way), is unheard of in extant
>> mammals, birds, and 'reptiles'.
>
>
> This may or may not be true for extant tetrapods; I don't know. But the skull 
> of _Triceratops_ apparently underwent this kind of transformation (including 
> erosion), as Horner &c explicitly point out:
>
> "The squamosal horns and nodes of _Pachycephalosaurus_ underwent
> a similar transformation to the epiparietal and episquamosal
> elements of _Triceratops_ [16:figure 2] that (1) grew from
> diminutive to large triangular-shaped ornaments; (2) eroded as
> they reduced dorsoventrally and (3) flattened and lengthened as
> they merged onto the edge of the parietal-squamosal frill."
>
> (BTW, this has nothing to do with the whole _Torosaurus_=_Triceratops_ 
> question.)
>
>
> Again, just a few things to think about.
>
>
> Cheers
>
> Tim
>
>
>
>
>                                         
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